Investigations on the innervation of the diaphragm in cats and rodents

Abstract In cats, rats and mice the diaphragm possesses an intrinsic plexus of nerve fibres which are concentrated in a narrow band situated approximately halfway between the edge of the central tendon and the peripheral attachment of the musculature. Contributions to the plexus originate in both phrenic and lower six intercostal nerves and both possess three categories of fibres. The first is composed of unmyelinated fibres which ramify on the walls of companion vessels. The second comprises myelinated fibres ranging from 6–10 μ in diameter and these have been classified as afferent components. The third category, forming the major part of the phrenic outflow but only a minor part of intercostal contributions, comprises inyelinated fibres which range from 10–16 μ in diameter and possess preterminal expansions. Phrenic fibres of the third category have been identified as efferent components because they terminate in motor end plates but the function of corresponding intercostal components cannot be defined conclusively. Quantitative studies at three distinct levels within each of ten phrenic nerves in cats, show an increase in the number of myelinated fibres of diameter 12–20 μ in the peripheral third of the nerve. This suggests that the phrenic nerve receives an accession of fibres in its peripheral third.